1. Field of the Invention
The present invention relates to a method for preventing a problem of collision between an optical system and the medium by suggesting a method for confirming the kind of the medium prior to its operation in a drive having a compatibility with respect to a method for storing a near field optical information using SIL and a method for storing a remote field optical information below the existing blu-ray disk.
2. Background Art
In accordance with the developments of various technologies in a field of optical storing equipments, the various optical storing equipments have been developed. The currently developed optical storing equipments are largely divided into those using a remote field method and the others using a near field method. Generally, one optical storing equipment supports one of the two methods. Recently, an optical storing equipment having a compatibility with the two methods was presented. The features of the optical storing equipments of a near-field method and a remote-field method will now be described.
FIG. 1 is a block diagram showing an optical pickup unit of the configuration of an optical storing equipment using a remote field method. The optical pickup unit includes a laser diode (101), a collimator lens (102), an optical separator (103), an objective lens (104), a cylindrical lens (105) and a photodiode (106).
FIG. 1 shows the configuration of the first generational optical pickup for a DVD, of which a light pathway is divided into a light transmitter before a light generated from the laser diode (101) is transmitted to the objective lens (104) and a light receiver of the photodiode (106) where a light condensed by the objective lens (104) is reflected in a disk and detects an optical signal.
The principle of the light transmitter is as follows. In general, the light generated from the laser diode (101) is emitted with an intensity distribution in an ellipse. As the objective lens (104) has a plane of incidence in a circle, the intensity distribution of the emitted light should be changed to be in a circle using a beam shaping prism in order to effectively condense a light to the objective lens (104).
As the light output from the laser diode (101) becomes a linear polarized light, an optical separator (103) is used in order to minimize the loss of light. The optical separator (103) is not a function required in the transmitter but used to detect a light reflected back from the disk. The laser diode (101) is applied so that the direction of its linear polarized light is parallel with a bottom face and the light is transmitted to the optical separator (103), the light polarized in the horizontal direction can have a transmittance with more than 97%.
If the transmitted light passes through a a./4 wavelength plate (not shown), a linear polarized light is changed into a circular polarized light by the wavelength plate and reflected by a reflection mirror for decreasing the height of a pickup and inputted to the objective lens (104).
The light input to the objective lens (104) is reflected by the disk (107) and the light beam reflected by the disk (107) is transmitted through the objective lens (104). The light reflected by the reflection layer of the disk (107) (with a high refraction rate in the air) has a low refraction rate, and a phase is changed by 180° in a frequency of a light. In this case, if a light polarized in a circle by the λ/4 wavelength plate is transmitted through the λ/4 wavelength plate again, it is changed into a linear polarized light which is perpendicular to the original polarized direction. In accordance with the characteristics of a polarized light separator (103), a light is effectively detected in the photodiode (106). In other words, a horizontally polarized light is mostly transmitted, but a vertically polarized light is mostly reflected.
In the development of current optical storing technology, CD, DVD and BD have been sequentially developed that, among other things, increase recording volume. A principle used to increase recording volume is to shorten a wavelength of a laser light or to increase a numerical aperture (N/A) of an objective lens. In a blue-ray disk, the method to shorten a wavelength is determined to reach a limit. The numerical aperture (N/A) of the objective lens is determined to be difficult to develop by the existing remote-field method more.
Recently, a near-field recording method using the newly developed SIL (solid immersion lens) may provide a technology for dramatically increasing recording volume. In the method, a hemispherical SIL made of a medium of which the refraction rate is considerably greater than 1 is disposed under an objective lens so that a substantial numerical aperture (n*NA) is greater than 1.
FIG. 2 is a cross-sectional view of an optical pickup unit of an optical storing device supporting a near-field method. In a device using a near-field method, the optical pickup includes a solid immersion lens (SIL) (202) contrary to an optical storing device using a remote-field method. The principle of an optical pickup using a near-field method having a solid immersion lens (SIL) (202) is as follows.
Referring to FIG. 2, if a laser is condensed to the solid immersion lens (SIL) (202) by the objective lens (201), the light is focused on the bottom surface of the solid immersion lens (SIL) (202) with a high refraction rate. A focused spot mirror is decreased in inverse proportion to the refraction rate of the solid immersion lens (SIL) (202) to obtain a spot less than a diffraction limit.
The spot is transmitted to a medium in the form of an evanescent light to heat a surface that stores a bit information by a method such that the area is magnetized in a specific direction by the heated domain layer magnetizing coil. In the method, a position in a horizontal direction is automatically controlled in a probe method by a floating head allowing for the recording and reproducing of information at high speed.
In general, an optical system consists of two lens. A free focus lens is loaded on a front surface of a floating head, corresponding to an objective lens (102) in a conventional optical magnetic disk device, and a hemisphere or a supersphere lens such as the solid immersion lens (SIL) (202). The solid immersion lens (SIL) (202) is made of a material with a high absolute refraction rate (n) like a glass to be mounted on a lower surface of a floating head. The recording reproduction system mainly uses an optical magnetic recording of a magnetic demodulation type such as a mini disk. In this method, a temperature of the recording layer in a beam spot is temporarily increased by an irradiation of the laser beam to record using a magnetic force limited to a coil above the Curie temperature where magnetization is easily lost.
What is to be considered important in developing a new type of optical storing device is a compatibility with existing methods. Users sometimes avoid using a new method which is not compatible with existing methods, and it is difficult to enter the current market. Accordingly, the problem to be considered is if the media used may be used in both newly developed and existing drives. Especially, since the distance between the bottom surface of a solid immersion lens (SIL) and a surface of the medium of an optical system used in a near-field method is merely tens of nm, if users do not have a way to see the currently inserted media exactly an optical system or a media may be damaged by collisions during between the drives and medium.
In order to avoid this situation, a new process is required for quickly identifying a media prior to operation of a compatible drive.